Efforts to reduce sulfur emissions in gaseous products resulting from the combustion of sulfur-containing fuels have been made of late in order to comply with environmental regulations. Many processes attempt to reduce or eliminate sulfur in fuels prior to the fuels combustion. Many processes propose the addition of compounds to the combustion zone for the purpose of reacting with and removing the gaseous sulfur compounds from the combustion products. However, other processes remove sulfur compounds from the gaseous combustion products by chemical reaction downstream of the combustion device.
U.S. Pat. No. 4,185,080 discloses a combustion gas desulfurization process when a powdered sorbent such as limestone or dolomite is added to a combustion zone and a portion of the solids collected from the flue gas is reactivated and returned for injection into or downstream of the combustion zone. An earlier U.S. Pat. No. 3,320,906 teaches mixing limestone with the coal being fed to a boiler with the flue gas then being passed to a wet scrubber for additional capture of sulfur compounds by calcined limestone.
One approach to chemical treatment to effect flue gas desulfurization is dry treatment when the treating reagent is a solid powder both entering and leaving the contacting zone. However, in order to be effective, most dry powders must be in a humidified atmosphere in order to promote the absorption of the sulfur compounds into the sorbent.
Further, it has been found that certain additives promote the absorption of sulfur dioxide, SO.sub.2, by these dry sorbents. Such sulfur promoters are used in a small amount relative to the main absorption material such as hydrated lime or dolomite primarily calcium base materials in order to enhance sulfur absorption. In this way, humidification and promotion, the absorption capability of such calcium base materials can be raised from about 50 percent SO.sub.2 removal to over 70 percent SO.sub.2 removal (at a Ca/S mole ratio of about 2/1).
Numerous site specific factors such as proximity to reagent source, space availabilty and extent of sulfur reduction required must enter into the selection of the process used at each combustion site. However, in each site some concerns remain the same, such as cost of reagents, cost of disposal and cost of absorbents. When using promoters such as alkali metal promoters and the like, it is necessary not only to consider the cost of the promoter in relation to the calcium based absorbent but also the cost of disposal. When using the most effective sodium and potassium based absorbents, disposal may become a problem since these materials must be "stabilized" or placed in a form where they will not enter the environment in active concentrations. It is necessary to lower the amount of such materials in disposed flyash particulate matter to below 11/2 percent by weight in order to avoid disposal problems.
It would therefore be of great benefit to provide a method for recovering absorption promoters from particulate flyash containing said promoters in order to reduce absorption expense and reduce disposal problems.
It is therefore an object of the present invention to provide a method for recovering absorption promoters from hydrate modified flyash generated during the flue gas desulfurization processes. Other objects will become apparent to those skilled in this art as the description proceeds.
We have now discovered that water soluble desulfurization promoters can be recovered from hydrate-modified flyash containing said promoters when said flyash is formed by combustion of sulfur-containing fuels and sulfur absorbing calcium and magnesium compounds are added to the flue gas after the combustion zone in the presence of such promoters. The process comprises collecting the particulate flyash in a particulate control device and separating said flyash into recovery and disposal streams then mixing the disposal flyash stream with water to form a slurry containing up to 30 percent solids and recycling the recovery stream to the flue gas for further desulfurization. The slurry formed by mixing the disposal flyash stream is then dewatered to a water content of from about 10 to about 50 percent by weight to form a wet cake and a recovered water stream containing the water soluble absorption promoters. The wet cake is then disposed and the recovered water soluble absorption promoters in the recovered water are again placed into contact with hot flue gases in the presence of calcium and magnesium sulfur absorbing compounds to form the hydrate modified fly ash which is then again recovered into two streams.
In general, recovery of sodium values from waste streams is known. Representative but non-exhaustive of such art is U.S. Pat. No. 3,870,871 wherein in a dual alkali process, alkali metal hydroxides are recovered in a scrubber which form sulfites or sulfates. Spent sorbent is oxidized such as from bisulfite to sulfate and then contacted with calcium oxide to regenerate sodium hydroxide. This patent teaches recovering and recycling water soluble additives in general from particulates to lower the sodium content and then disposing of the solubilized waste. U.S. Pat. No. 3,920,795 shows dewatering of calcium containing slurry. U.S. Pat. No. 4,385,039 removes sulfur oxides with solid sorbents using alkali metals and recycle liquids to dissolve sorbents then filtering and regenerating.